Heated semiconductor bonding tool

ABSTRACT

A high-efficiency heated bonding tool having a heat sink with a plurality of recesses therein for receiving a replacable heating element and a bonding nib, at least one recess comprising a helical thread into which is fitted a helical coil heating element wound as a spring having a diameter which would cause an interference fit with the helical thread, but when flexed to fit the helical thread provides a radial force for tight surface-tosurface engagement with the helical thread.

United States Patent 1 Feb. 8, 1972 Angelucci [541 HEATED SEMICONDUCTOR BONDING TOOL [72] Inventor: Thomas L. Angelucci, Cherry Hill, NJ.

[73] Assignee: Kulicke and Sofia Industries, Inc., lndustrial Park, Fort Washington, Pa.

[22] Filed: June 16, 1969 21 Appl. No.: 833,408

[521 US. Cl ..219/85, 29/4701, 29/4975,

219/143, 219/238, 228/3, 228/51, 279/76 [51] Int. Cl ..H05b 1/00, B23k 21/00, B23k 3/00 [58] Field of Search ..219/221, 227-240,

[56] References Cited UNITED STATES PATENTS 3,136,878 6/1964 Staller ..219/239 1,008,922 11/1911 MacLagan ..219/238 1,179,476 4/1916 Thomas 279/76 X 1,604,069 10/ l 926 Owen ..219/ 143 1,708,995 4/1929 Abbottm, ..219/239 3,101,635 8/1963 Kulicke 29/497.5 X 3,358,897 12/1967 Christensen ....219/23O X 3,384,283 5/1968 Mims ..29/470.1 X

FOREIGN PATENTS OR APPLICATIONS 901,872 11/1944 France ..219/236 617,414 8/1935 Germany ....219/337 630,098 5/1936 Germany ....219/3 15 88,226 5/1921 Switzerland ..219/239 Primary ExaminerA. Bartis Attorney-John B. Sowell [57] ABSTRACT A high-efficiency heated bonding tool having a heat sink with a plurality of recesses therein for receiving a replacable heating element and a bonding nib, at least one recess comprising a helical thread into which is fitted a helical coil heating element wound as a spring having a diameter which would cause an interference fit with the helical thread, but when flexed to fit the helical thread provides a radial force for tight surfaceto-surface engagement with the helical thread.

9 Claims, 8 Drawing Figures PAIENIEMB '8 m2 SHEET 1 OF 3 INVENTOR. Thomas L. Angelucci 9&4; Qymz ATTORNEY.

PAYENTEMB a 1912 3,541, 4

SHEET 2 [IF 3 Fig 6 INVENTOR. Thomas L. Angelucci Y wymzz ATTORNEY.

PAIENTEDFEB 'e 1972 SHEET 3 UF 3 INVENTOR.

Thomas L. Angelucci 9%:a Gym/Z ATTORNEY.

1 HEATED SEMICONDUCTOR BONDING TOOL BACKGROUND OF THE INVENTION In the manufacture of semiconductor devices it is known practice to heat the bonding tool and/or the semiconductor carrier to facilitate making thermocompression bonds. Such bonds are made when gold or similar wires or leads are simultaneously heated and pressed into a terminal or an electrode of a semiconductor device. It is generally desireable to heat the tip of the bonding nib to as high a temperature as possible within the range of acceptable bonds to insure good molecular adhesion and to increase the speed of the operation. Heretofore, attempts to heat the tip of a bonding tool near its optimum bonding temperature have resulted in rapid deterioration of the heaters, variation in tip temperatures and rapid wearing out of the bonding nibs or heaters. Attempts to heat the bonding nibs by passing electrical current through the bonding nibs have not proven to be generally acceptable because of overheating and excess oxidation.

SUMMARY OF THE INVENTION It is the primary object of the present invention to provide an efficient and economical bonding tool having a disposable and replaceable heating element.

It is another object to provide a heated bonding tool having a heat sink which maintains the temperature of the bonding nib uniform and within 25 C. of the heating element.

Accordingly, there is provided a small integral body heat sink having a first recess therein to receive a replaceable bonding nib in surface contact near the tip of the nib, and in close proximity to the nib a second recess in the heat sink for receiving a heating element, both said bonding nib and said heating element being urged into contact with large interface surfaces of the heat sink to increase the heat transfer therebetween.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevation of a preferred embodiment wire bonding tool.

FIG. 2 is an enlarged perspective view of the bonding tool of FIG. 1.

FIG. 3 is an enlarged front elevation taken at lines 3-3 of FIG. 1.

FIG. 4 is a partial section in side elevation taken at lines 4- 4 of FIG. 3.

FIG. 5 is a side elevation of a modified embodiment wire bonding tool.

FIG. 6 is a section in plan view taken at lines 6-6 of FIG. 5.

FIG. 7 is an enlarged perspective view of another modified embodiment wire bonding tool.

FIG. 8 is an enlarged elevation of a modified embodiment beam lead bonding tool.

DESCRIPTION OF THE PREFERRED EMBODIMENT The preferred embodiment heated bonding tool 1, shown in FIGS. 1 to 4, is adapted to be connected to a wire bonding machine of the type generally used for thermocompression bonding fine gold wires. Bonding tool 1 comprises a heat sink 2, preferably machined from a block of high-temperature nickel steel having good heat conductive qualities. The heat sink 2 can be made from several parts if they are connected together by a good heat conductor at the connecting interfaces.

A first recess 3 in the heat sink 2 has an Open vee shape 4, which permits rapid replacement and critical alignment of a bonding nib 5 should it become clogged, worn or otherwise require replacement. Bonding nib 5 is preferably made short, extending below the heat sink only as far as necessary to provide visual clearance and working clearance over the semiconductor device (not shown). Bonding nib 5 preferably engages the sides of vee 4 in vertical line-to-line contact, thus, assuring fixed seating of the nib 5, even through the nib 5 or the vee 4 may be subject to thermal distortion.

A second recess 6 in the heat sink 2 is located in close proximity to the first recess 3 and has a large area connection 7 therebetween to provide optimum heat transmission. The second recess 6 is cylindrical in shape and is provided with helical threads 8 to receive a cylindrical-shaped round wire or rod helical coil heating element 9 of the known sheath-type. The pitch of the helical threads is slightly greater than the pitch of the coil of the heating element 9 causing the coils to be biased apart when installed in the threads. The diameter of the coil of the heating element 9 is preferably slightly larger than the inside diameter of the helical threads 8, however, the end 11 of coil 9 may be grasped in a bifurcated tool (not shown) and screwed into the helical threads in a manner which tends to contract the coil radially. Functionally, coil 9 is a hollow spring which expands both axially and radially in the recess 6 to provide a tight compression fit therewith during operation. Heating element 9 is a performed miniature rodtype heating element operated at temperatures varying from 250 to over 700 C. which causes the surface of the heating element and the recess 6 to become tightly fixed together at their mating surfaces. Coil 9 may be removed from the recess 6 by grasping an end and screwing the coil in a direction which causes radial contraction. If the conductors in the coil is burned out or otherwise being replaced, the round wire or rod is strong enough in tension to be pulled out of the recess 6, thus, distorting and unraveling the coil 9.

A third recess 12 in the heat sink 2 is a housing or mount for the nib retainer 13 which urges the bonding nib 5 into engagement with the heat sink 2. A cap or heat 14 on a hollow cylinder 15 is both slidably and rotatably mounted in the third recess 12. A tension spring 16 stretched between pins 17, 18 urges cylinder 15 into the recess 12, however, the back surface 19 of the head 14 engages the bonding nib 5 and is prevented from engaging the heat sink. A screwdriver slot 21 in the head 14 enables an operator to easily rotate the flat 22 on the head 14 to position adjacent and parallel to the bonding nib 5, permitting the bonding nib to be easily replaced. Heat sink 2 may be heated to a temperature within 10 C. of the heating element 9, thus, creating a condition which would destroy an ordinary spring 16. Spring 16 is shielded by hollow cylinder 15, and is further cooled by air flowing past pins 17, 18 at the open ends of recessed 12. An inconel alloy spring capable of withstanding high temperatures has been found to be operable under these temperature conditions.

A nib stop 23, such as a rivet having a head 24 overlying a portion of the open vee 4, is connected to the heat sink 2. Nib stop 23 serves to position and critically align the bonding nib 5 during replacement, and further serves as an up-stop for compressive loading the bonding nib 5.

A long thin-wall hollow cylinder 25 connected to the top of the heat sink 2 serves as a heat choke. The upper end of the heat choke 25 is connected to an insulating mounting tube 26 by a band 27.

In normal operation the heating element 9 is connected to a variable transformer (not shown) or other variable voltage source. After a short period of operation the bonding tool 1 will stabilize at a fixed temperature. Surrounding ambient temperature changes do not greatly effect the stabilization temperature because the major portion of the heat loss is by radiation. To avoid excessive radiation heat losses, the external surfaces of the heat sink are preferably kept small. It has been found that a heat sink which substantially surrounds both the bonding nib S and the heating element 9 is large enough. Recess 6 is made relatively large to provide for a high wattage heater and to provide a large area for heat transfer. The higher the wattage input, the greater the temperature of the heat sink and the bonding nib. Placing the heating element 9 inside recess 6 greatly diminishes the radiation heat loss from the heater 9. External surfaces of the heat sink 2 are not as hot as the heating element 9, thus, the radiation losses from the heat sink 2 are less than would have occurred with external placement of the heating element. Connection 7 between recess 3 and recess 6 occurs at a large cross-sectional area of the heat sink which enables the transfer of heat therebetween with a negligible drop in temperature. Since the heating element 9 is in closed proximity to the open vee 4, there is negligible heat loss at the connection 7, and the temperature of the surface of the vee 4 is maintained very close to that of the recess 6. A large portion of the bonding nib 5 is exposed to the radiating surfaces of the vee 4, thus, enabling the bonding nib 5 to be maintained within at least 25 C. of the recess 6 even at the highest operating temperatures.

A modified embodiment heated wire bonding tool is shown in FIGS. 5 and 6 wherein heat sink 26 supported by heat choke 27 comprises a cylindrical tube 28 attached to a body 29 by silver solder 30 or similar means. Bonding nib 5 is removably mounted in a first recess 31 in close proximity of heating element 32 which is removably mounted in a second recess 33. Nib retainer 34 is mounted in a third recess 35 in the heat sink 26 and comprises a cylindrical shell 36 housing a ball 37 and spring 38. The nib retainer 34 may be fixed to the heat sink by an interference or press-fit, or other well known means.

Nib stop 39 comprises an open retaining cup 41 connected to the heat sink 26 to entrap a ring-shaped washer or jewel 42 having a highly polished aperture. Synthetic sapphire watchmaker jewels have been found satisfactory for this purpose because they tend to center the wire in the bonding nib, thus, reducing friction between the wire and the bonding nib as well as making the loading operation easier.

Recess 33 is shown to be a smooth cylindrical bore for receiving a commercially available cartridge-type heater 32. While this type heater is not as efficient as the illustrated preferred embodiment heater, it may be used for operations not requiring high temperature or maximum device visibility.

FIG. 7 shows a modified embodiment heat sink 43 which is adapted to accept a removable coil-type heating element (not shown) in external recess 44. It will be understood that the coil-type heater (as shown in FIG. 8) should be smaller in diameter than the diameter of the recess 44 causing the coil heater to exert a radially inward force on the heat sink 43. As already explained, placing the heating coil on the external surface increases radiation heat losses, however, where a large heating coil is to be placed on a small heat sink body and the operating temperatures are very high, heating coils of this type may be employed even though shields may be required. The nib stop 45 and nib retainer 46 are similar to those explained with regard to FIG. 2.

FIG. 8 shows a beam lead bonding tool 47 having a removable coil-type heating element 48 fitted in external recess 49 of the heat sink 51. Bonding nib 52 is preferably removably press-fitted in the heat sink 51, however, such bonding nibs do not clog as easily as wire bonding nibs and they may also be reworked to restore their working faces so that the ease of removal of this bonding nib is not as important as the requirement for wire bonding nibs. The heat choke and holder 53 for mounting bonding tool 47 are vertically aligned with the nib 52, which permits the bonding tool to be connected directly to heads which wobble the tool about the working face 54.

It is apparent that beam lead bonding nib 52 could be installed in bonding tools similar to those shown in FIGS. 1 to 7, however, if it is desirable to have the axis of the holder or heat choke vertically aligned with the bonding nib, an offset shaped holder, similar to the one shown in FIG. 1, could be used.

Having described the preferred embodiment heat sink adapted to mount a removable heating element in close proximity to a bonding nib, it can be seen that heat is transmitted from the heating element to the bonding nib through only two interfaces on the heat sink proper. The areas of the heat sink through which heat is conducted, as well as the heat transfer interfaces, are large relative to the size of the surfaces which radiate heat to the surrounding environment. The heat source is located close to the bonding nib so that the heat gradient or temperature decrease is minimized. The heat gradient or temperature loss between the heating element and the bonding nib has been measured to be as small as C. and did not exceed 25 C. even when operated around 600 C. Prior art heated bonding tools of the type most commonly used in the United States are not known to be capable of operating as high as 600 C. The heat gradient between the heater and the bonding nib of such prior art devices has been measured to be in excess of C. when the bonding nib was only 320 C. The gradient would increase at higher bonding nib temperatures.

The ability of the present heated bonding tool to be operated at temperatures in excess of 600 C. permits thermocompression bonds to be made on unheated semiconductor devices with a continuously heated and stable-temperature bonding tool. Higher temperatures, approaching the limit of the heater to operate without burning out, are obtainable with the present novel structure.

While a preferred embodiment and three modifications thereof have been described for purposes of illustration, it is to be understood that there may be other modifications within the general scope of the invention.

I claim:

1. A heated bonding tool for the manufacture of semiconductor devices comprising: a heat sink, a first recess in said heat sink for receiving a removable bonding nib therein, means mounted on the heat sink for urging the bonding nib into engagement with the heat sink, a second recess in said heat sink comprising a helical thread for receiving therein in surface to surface contact a removable flexible round wire heating element formed as a helical coil, said second recess being located in close proximity to said first recess in the heat sink, a round wire helical coil heating element formed as a flexible helical coil of greater diameter than the helical threads, said helical coil being radially compressed to conform to the shape and size of the helical threads when installed therein providing a radial force engaging said helical coil heating element in surface of surface compression fit with said helical threads, and a heat choke connected to and extending from said heat sink for limiting heat transfer from the heat sink and mounting the bonding tool to a bonding machine.

2. A heated bonding tool as set forth in claim I, wherein the round wire helical coil heating element is tightly held against the helical threads through radial forces exerted by the helical coil acting as a radial spring to provide a compression fit with the heat sink.

3. A heated bonding tool as set forth in claim 2, wherein the pitch of said helical coil heating element as formed is less than the pitch of the helical thread for providing an axial compression fit and separation of the coils of the heating element.

4. A heated bonding tool as set forth in claim I, wherein said first recess comprises an open face groove and said means urging the bonding nib into engagement with the heat sink comprises a nib retainer having a clamp head ,rotatably mounted on the heat sink for urging the bonding nib into engagement with the heat sink.

5. A heat bonding tool as set forth in claim 4, wherein said clamp head is urged into engagement with the bonding nib by a coil spring located in a third recess in the heat sink.

6. A heated bonding tool as set forth in claim 5, wherein said nib retainer further includes a hollow cylinder attached to the clamp head which is slidably and rotatably mounted therewith in the third recess in the heat sink, a flat on the clamp head which is rotatable to a first position wherein the flat is disposed parallel to the axis of the bonding nib and the open face groove to permit loading and unloading of a bonding nib, said flat on the clamp head being rotatable to a second position wherein the flat is disposed transverse to the axis of the bonding nib and the open face groove to permit the clamp head to retain the bonding nib in the first recess of the heat sink.

7. A heated bonding tool for the manufacture of semiconductor devices comprising: a heat sink comprising an integral block of heat conductive metal, a first recess in said heat sink forming a first interface surface on the heat sink, a bonding nib tightly fitted against said first interface surface, a second recess in said heat sink in close proximity to said first recess forming a second interface surface, said second interface surface comprising a helical thread adapted to accept a removable heating element therein, a cylindrical-shaped flexible round wire helical coil heating element formed as a flexible helical coil of different diameter than the helical thread, said helical coil being radially flexed as a spring to conform to the shape and size of the helical thread when installed therein providing a radial force for engaging said helical coil heating element tightly fitted in surface to surface contact with said helical thread for transmitting heat through the two interface surfaces on the heat sink to the bonding nib, and heat choke means on the heat sink for mounting the bonding tool to a bonding machine.

8. A heated bonding tool as set forth in claim 7, wherein said helical coil heating element is flexible in a radial direction to permit the heating coil to change its diameter and to be screwed onto the helical thread, and is sufficiently strong as a round wire in tension to permit removal from the helical thread by deforming and unraveling the helical coil heating element.

9. A heated bonding tool as set forth in claim 7 wherein said helical coil heating element is further provided with a screw end formed as a radial bisector of the end of the coil, said screw end providing means for screwing said coil into said helical thread and simultaneously contracting the diameter of said helical coil. 

1. A heated bonding tool for the manufacture of semiconductor devices comprising: a heat sink, a first recess in said heat sink for receiving a removable bonding nib therein, means mounted on the heat sink for urging the bonding nib into engagement with the heat sink, a second recess in said heat sink comprising a helical thread for receiving therein in surface to surface contact a removable flexible round wire heating element formed as a helical coil, said second recess being located in close proximity to said first recess in the heat sink, a round wire helical coil heating element formed as a flexible helical coil of greater diameter than the helical threads, said helical coil being radially compressed to conform to the shape and size of the helical threads when installed therein providing a radial force engaging said helical coil heating element in surface of surface compression fit with said helical threads, and a heat choke connected to and extending from said heat sink for limiting heat transfer from the heat sink and mounting the bonding tool to a bonding machine.
 2. A heated bonding tool as set forth in claim 1, wherein the round wire helical coil heating element is tightly held against the helical threads through radial forces exerted by the helical coil acting as a radial spring to provide a compression fit with the heat sink.
 3. A heated bonding tool as set forth in claim 2, wherein the pitch of said helical coil heating element as formed is less than the pitch of the helical thread for providing an axial compression fit and separation of the coils of the heating element.
 4. A heated bonding tool as set forth in claim 1, wherein said first recess comprises an open face groove and said means urging the bonding nib into engagement with the heat sink comprises a nib retainer having a clamp head rotatably mounted on the heat sink for urging the bonding nib into engagement with the heat sink.
 5. A heat bonding tool as set forth in claim 4, wherein said clamp head is urged into engagement with the bonding nib by a coil spring located in a third recess in the heat sink.
 6. A heated bonding tool as set forth in claim 5, wherein said nib retainer further includes a hollow cylinder attached to the clamp head which is slidably and rotatably mounted therewith in the third recess in the heat sink, a flat on the clamp head which is rotatable to a first position wherein the flat is disposed parallel to the axis of the bonding nib and the open face groove to permit loading and unloading of a bonding nib, said flat on the clamp head being rotatable to a second position wherein the flat is disposed transverse to the axis of the bonding nib and the open face groove to permit the clamp head to retain the bonding nib in the first recess of the heat sink.
 7. A heated bonding tool for the manufacture of semiconductor devices comprising: a heat sink comprising an integral block of heat conductive metal, a first recess in said heat sink forming a first interface surface on the heat sink, a bonding nib tightly fitted against said first interface surface, a second recess in said heat sink in close proximity to said first recess forming a second interface surface, said second interface surface comprising a helical thread adapted to accept a removable heating element therein, a cylindrical-shaped flexible round wire helical coil heating element formed as a flexible helical coil of different diameter than the helical thread, said helical coil being radially flexed as a spring to conform to the shape and size of the helical thread when installed therein providing a radial force for engaging said helical coil heating element tightly fitted in surface to surface contact with said helical thread for transmitting heat through the two interface surfaces on the heat sink to the bonding nib, and heat choke means on the heat sink for mounting the bonding tool to a bonding machine.
 8. A heated bonding tool as set forth in claim 7, wherein said helical coil heating element is flexible in a radial direction to permit the heating coil to change its diameter and to be screwed onto the helical thread, and is sufficiently strong as a round wire in tension to permit removal from the helical thread by deforming and unraveling the helical coil heating element.
 9. A heated bonding tool as set forth in claim 7 wherein said helical coil heating element is further provided with a screw end formed as a radial bisector of the end of the coil, said screw end providing means for screwing said coil into said helical thread and simultaneously contracting the diameter of said helical coil. 